The invention relates to the field of reducing primary injurious substances generated in internal combustion engines by means of a catalyst. The invention relates, in particular, to a process for reducing nitrogen oxides (NOx) on a SCR catalyst, disposed in the exhaust gas system of an internal combustion engine, with ammonia (NH3) which is preferably introduced into the exhaust gas flow before the catalyst. The invention further relates to a device for introducing ammonia (NH3) into the exhaust gas flow of an internal combustion engine for reducing nitrogen oxides (NOx) contained in the exhaust gas flow on a SCR catalyst comprising a NH3 source, a supply line for introducing the NH3 into the exhaust gas flow and a dosing device.
Apart from carbon monoxide (CO) and hydrocarbons (HC), in particular the nitrogen oxides (NOx) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, in particular Diesel engines. The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of Diesel engines due to an oxygen excess. For this reason, for the reduction of the nitrogen oxide emission in Diesel engines a selectively operating SCR (Selective Catalytic Reduction) catalyst has been developed in which, in the presence of an added reducing agent, namely ammonia (NH3), the expelled nitrogen oxides are reduced to N2 and H2O.
This type of reducing nitrogen oxide emissions in stationary Diesel engines has been found to be useful. In these stationary installations in the exhaust gas system of the internal combustion engine is disposed a SCR catalyst and the NH3 to be added to the exhaust gas flow before the SCR catalyst is added by injection. In such installations the NH3 is supplied in the form of a gas or aqueous solution. In the case in which NH3 is supplied as an aqueous solution, a thermolytic splitting takes place in the exhaust gas flow, or in the SCR catalyst, in order to release the NH3 necessary for the reduction of nitrogen oxides. The introduction of the reducing agent takes place via a dosing device which is adjusted as a function of the anticipated NOx quantity in the exhaust gas flow of the Diesel unit. When using NH3 either as the gas or as aqueous solution, the use of skilled personnel is required since associating with these substances is not without danger with respect to their handling as well as also with respect to their toxicity.
Due to the not undangerous association with NH3 in gaseous form or as an aqueous solution, it has become customary to provide the NH3 required for the NOx reduction by introducing aqueous urea solution into the exhaust gas flow. The thermohydrolytic splitting with the liberation of NH3 as reducing agent takes place through the heat of the exhaust gas flow or of the catalyst. In addition to the difficulties relating to the process of the unchecked feed of this reducing agent in the form of an aerosol, a further disadvantage entailed in obtaining NH3 in this way is the formation of undesirable byproducts such as, for example, isocyanic acid. The fact that the freezing point of such a reducing agent is approximately xe2x88x9213xc2x0 C., moreover, stands in the way of using aqueous urea solutions for the NOx reduction of Diesel engine exhaust gases in mobile units, for example in utility vehicles or passenger motor vehicles. The winter-worthy use of this reducing agent is therefore only possible by mixing in additives which decrease the freezing point. However, the conversion of these additives in the SCR catalyst can lead to the emission of undesirable secondary injurious substances.
It is furthermore necessary to carry along a relatively large quantity of aqueous urea solution since the urea necessary for the reduction is only present in the aqueous solution at a ratio of at best 1:3 with respect to the H2O.
The use of NH3 carried along in pressure tanks or as aqueous solution for removing nitrogen in mobile Diesel units is not possible in the event of an accident due to the danger inherent in these substances.
From DE 34 22 175 A1 as well as DE 42 00 514 A1 processes are known which relate to the xe2x80x9cjust-in-timexe2x80x9d production of NH3 for the reduction of NOx. The superior concept evident in these patents comprises using specific substances which thermolytically split off NH3, the handling and toxicity of which are quite safe, in order to split off the required quantity of NH3 in accordance with the particular requirements and to inject it into the exhaust gas flow. The generation of NH3 takes place by heating such an NH3-splitting compound, for example ammonium carbamate. The adaptation to the particular operating state of the motor, as described in DE 34 22 175 A1, takes place thereby that by controlling the calorific power, which for example acts on the carbamate, the NH3 generation and thus the NH3 quantity added to the exhaust gas flow can be regulated.
Such a regulatable NH3 generator comprises essentially a storage tank for the compound splitting off NH3, a decomposition chamber in which the action of the heat takes place, as well as a takeout line or supply line for the NH3-containing gas. For heating the decomposition chamber, in particular electric heating [systems], for example resistance heating bodies or infrared radiators are provided.
The use of this prior known process or this prior known device is suitable for stationary installations which in general are only subject to few load changes and even then only to predetermined load changes, however, for deployment in mobile use, such as for example in utility vehicles or passenger vehicles, the use of this prior known technology is not appropriate since the reaction time of the system is too slow and thus the system too inert in order to be able to do justice to the unexpected motor load changes occurring in rapid succession in street traffic with correspondingly varying NOx emissions. If the operating state of the motor is acquired at a specific time and, based on this acquisition, a specific NH3 dosing is calculated, the substance splitting off the NH3 must first be heated in order to produce the NH3-gas mixture. However, in street traffic, in particular in city traffic, an engine is subject to continuous and unexpected load changes such that the quantity of NH3 which is lastly introduced does not match the operating state of the engine which has changed in the meantime. If an insufficient quantity of NH3 is introduced, the NOx reduction can only be carried out to a limited extent. If the NH3 is introduced in excess, unconsumed NH3 exits the catalyst.
Building on the discussed prior art, the invention is therefore based on the task of proposing a process as well as a device for the NOx reduction of exhaust gases of an internal combustion engine using a SCR catalyst, which is not only suitable for mobile application but with which the above listed disadvantages are avoided.
This task is solved according to the invention through a process according to the species thereby that the process comprises the following steps:
initial heating of a substance or substance mixture at ambient temperature and capable of thermolytically splitting off NH3 disposed in a pressure-tight receptacle (converter) to that temperature range at which the splitting-off of NH3 takes place (splitting-off temperature range),
maintaining the splitting-off temperature range until an internal pressure builds up in the converter,
intermediately storing the split-off NH3 or NH3-gas mixture and, after reaching a predetermined reduction gas pressure in the NH3 storage,
acquiring engine operating characteristics according to which the NOx content in the exhaust gas flow can be calculated, based on which characteristics subsequently the determination of the NH3 dosing, necessary for the reduction of the calculated NOx content, takes place,
injection of a predetermined dose of the stored NH3 into the exhaust gas flow of the internal combustion engine with the last two process steps being repeated with optional frequency during the operation of the engine.
The task is further solved according to the invention thereby that as the NH3 source a heatable pressure-tight converter is provided in which is disposed a substance or a substance mixture splitting off NH3 thermolytically and that the dosing device is preceded by an NH3 store for the intermediate storage of the NH3split off from the substance through the addition of heat, which dosing device is acted upon with control signals by a control unit processing the engine characteristics and determining on that basis the NOx output.
By providing a process which makes possible the use as NH3 precursor of a substance or a substance mixture which, in terms of its handlability and its toxicity is unobjectionable, and which, by building up an internal pressure in the converter, utilizes the establishment of an equilibrium state for the termination (in time) of a further NH3 splitting, a process is created with which only a specific NH3 quantity is made available. At any given time a specific NH3 quantity is therefore available which can be injected into the exhaust gas flow in order to reduce NOx. The intermediately stored quantity of NH3 is so small that even in the case of a hypothetical destruction of the converter in the event of an accident it can be classified as unobjectionable due to its rapid mixing with ambient air.
By acquiring the engine operating characteristics and, if appropriate, the concentrations of exhaust gas components, calculation of the NOx mass flow in the exhaust gas becomes possible and, based on it, the calculation of the NH3 quantity. Since a sufficient quantity of NH3 is intermediately stored, a process is created which permits the NH3 output quasi-simultaneously with the NH3 determination. The process according to the invention as well as the device according to the invention are therefor primarily suited for the NOx reduction of mobile Diesel units subject to unpredictable load changes.
The invention combines the advantages of introducing the NH3 as gas into the exhaust gas flow, the carrying along of substances or substance mixtures which, with respect to their handling and toxicity are unobjectionable at ambient temperature, as well as making immediately available the required NH3. The process according to the invention as well as the device according to the invention are suitable for use of substances, splitting off NH3 substantially free of residues and decomposable thermolytically, such as for example ammonium carbamate (NH2CO2NH4) as well as for the use of substances reversibly NH3-absorbing/desorbing and thus splitting off NH3 such as for example of an iron (II) ammine sulfate.
For the initial heating as well as for maintaining the splitting-off temperature range of the converter, heat generated during operation of the internal combustion engine, is used and it is useful to assign to the converter a heating coil which is connected in particular when using ammonium carbamate to the cooling water circulation of the internal combustion engine. During operation of the internal combustion engine, the cooling water is at a temperature between 80 and 110xc2x0 C., which corresponds to an internal converter pressure of approximately 6.5-8 bars when attaining a state of equilibrium. The converter is therefore implemented so as to be suitably pressure-tight for operation at such internal pressures. It is advisable to take a safety margin into account.
An embodiment example provides that the intermediate storage of the split-off NH3 takes place in the converter itself. In a further embodiment example as the intermediate store a separate NH3 store is provided whose storage pressure is below the operating pressure of the converter.
For better dosing of the NH3 quantity to be removed, the converter usefully comprises a pressure-reducing valve. As the dosing device usefully a timed valve is provided.
A further development provides that the maximum NH3 dosing is slightly smaller than that NH3 dosing determined to be optimal According to such an implementation it is avoided that, in spite of the quasi-simultaneous NH3 introduction, upon an abrupt load change potentially non-converted NH3 exits the rear side of the SCR catalyst. To a limited extent, an emission of excess NH3 could also be reduced through an oxidation catalyst succeeding the SCR catalyst.
The converter comprising the substance splitting off NH3 can be connectable by means of rapid fasteners to the heating means provided for heating and maintaining the temperature as well as to the NH3 supply line. In this way the rapid exchange of the converter can be carried out, if the substance splitting off NH3 contained therein or the substance mixture splitting off NH3 contained therein is consumed. It is further possible to provide that the converter is comprised of a heating unit and a reaction receptacle with the reaction receptacle being detachable from the heating unit. The heating unit, connected for example to the cooling water circulation, remains in the motor vehicle such that only the reaction receptacle proper is exchangeable.